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Highlights from the Science family of journals

Highlights from the Science family of journals

Highlights from the Science family of journals

Highlights from the Science family of journals

Highlights from the Science family of journals

Highlights from the Science family of journals

Highlights from the Science family of journals

Highlights from the Science family of journals

Highlights from the Science family of journals

Highlights from the Science family of journals

Science is a social process. Discoveries—even by supposed “lone” geniuses—do not become knowledge until the findings are shared with the scientific community, to be vetted, challenged, and expanded on. Since the birth of the digital age, this discourse has become much more apparent, as communication outlets such as social media, blogs, and websites such as PubPeer and Retraction Watch allow the scientific community to discuss new findings. Further, the availability of greater amounts of data that support findings allows peers to redo key analyses to confirm or report discrepancies. This week, Science is announcing two changes—one to accelerate the conversation about papers and another to ease deposition of supporting data—that will facilitate the evaluation of research findings across the wide scientific enterprise.

Highlights from the Science family of journals

Highlights from the Science family of journals

Highlights from the Science family of journals

Atoms are dependably uniform—you would have a hard time, for instance, telling one rubidium atom from another. Many atomic properties have been measured to extraordinary precision: The frequency of an electronic transition in a cesium atom, for example, is used to define the unit of time. With

Organic Chemistry Linking nitrogen to the carbons in aryl rings is a common need in pharmaceutical chemistry. To do so, the carbon usually has to be adorned ahead of time with a reactive group, such as bromide. Paudyal et al. report a rhodium-catalyzed reaction that substitutes aryl C-H bonds with nitrogen groups directly to produce a wide variety of aromatic amines. Cleavage of a N-O bond in the nitrogen-bearing precursor drives the reaction with no need for an external oxidant. All of this can be done for both intermolecular and intramolecular couplings. Science , this issue p. [1144][1] [1]: /lookup/doi/10.1126/science.aaf8713

Actinide Chemistry You've probably heard of uranium and plutonium. Americium (Am) is less widely discussed outside chemistry circles, but the separation of this heavier radioactive element from nuclear waste streams is a major goal of fuel reprocessing research. The trouble is that trivalent Am ions are hard to tease apart from similarly charged lanthanide ions. Dares et al. now show that terpyridyl ligands appended to an electrode can promote the oxidation of trivalent Am ions to the pentavalent and hexavalent states (see the Perspective by Soderquist). These more highly charged ions should be easier to isolate for the subsequent use of the Am in next-generation nuclear reactors. Science , this issue p. [652][1]; see also p. [635][2] [1]: /lookup/volpage/350/652?iss=6261 [2]: /lookup/volpage/350/635?iss=6261

Inorganic Chemistry In chemistry, the term “aromatic” denotes the energy stabilization associated with electrons being shared among atoms in a ring. Benzene is the best-known aromatic compound, although numerous related hydrocarbons also manifest the property. Velian and Cummins now report a rare instance of an inorganic aromatic compound: a negatively charged pentagonal ring composed of three nitrogen and two phosphorus atoms. Science , this issue p. [1001][1] [1]: /lookup/doi/10.1126/science.aab0204

Organic Chemistry Numerous compounds in pharmaceutical research have carbon-nitrogen bonds, and chemists are always looking for ways to make them more efficiently. Gui et al. present a method that links the carbon in an olefin to the nitrogen in a nitroaromatic compound (see the Perspective by Kurti). Nitroaromatics are readily available, and the method tolerates a wide range of other chemical groups present on either reacting partner. Science , this issue p. [886][1]; see also p. [863][2] [1]: /lookup/volpage/348/886?iss=6237 [2]: /lookup/volpage/348/863?iss=6237

Chemistry The enormous quantity of energy released through fission of the uranium (U) nucleus has lent the element a widespread, intimidating reputation. At the lower energies prevailing in conventional chemistry, where electrons rather than neutrons are exchanged, U remains an intriguing target of

Chemistry The abstract prospect of using a circle to derive a square has intrigued scholars of geometry for centuries. Segawa et al. have now used a square to derive a circle. The arena in this case was chemistry, rather than mathematics, and the task at hand was to assemble a molecular ring composed in turn of 12 benzene rings, linked to one another at the diametrically opposed 1 and 4 positions. Previous efforts had yielded small quantities of this [12]cycloparaphenylene in a mixture of variously sized relatives bearing 9 through 18 benzenes. The key to a more efficient route was a building block linking two benzene rings at an approximate right angle through an intervening cyclohexane diol derivative. By thoroughly optimizing a catalytic nickel system, the authors succeeded in linking four such building blocks in a single reaction medium to form a square with the cyclohexane rings at the corners and two benzenes along each edge. Dehydration/aromatization of the corner rings then honed the square into the circular targeted product in sufficient yield to afford a crystal structure. Angew. Chem. Int. Ed. 50 , 10.1002/anie.201007232 (2011).

Chemistry A light-powered chemical reaction ultimately could help turn shale gas into plastic. When hydrocarbons such as butane come out of the ground, the carbon atoms in each molecule are connected by single bonds. Chowdhury et al. show at small scale that a soluble rhodium carbonyl catalyst, activated by light, efficiently snips hydrogen atoms off the molecules to create the more reactive carbon-carbon double bonds needed to make products such as plastics. Past efforts to remove hydrogen photochemically without creating by-products tended to be inefficient, but the authors suspected that flask walls might have been partly responsible by blocking some of the light. Using thin-walled glass flasks and an additive that activated the catalyst, the authors achieved on the order of 100 turnover cycles per hour for a range of different hydrocarbons. Angew. Chem. Int. Ed. 53 , 10.1002/anie.201402287 (2014).

Interfacial Chemistry Water behaves differently at interfaces—where it meets the air, or a solid surface—than it does in the middle of the liquid. Past laboratory studies of this phenomenon have mainly focused on still samples, despite the fact that in natural settings such as rivers and rain

Chemistry Chemical reaction trajectories tend to be largely confined to a single electronic potential energy surface. Occasionally, though, two surfaces intersect when vibrations on the lower surface become vigorous enough to match the energy level of the higher one. Such conical intersections (so

CHEMISTRY One advantage enzymes tend to have over synthetic catalysts is their capacity as very large molecules to create an environment that fully surrounds a substrate and thereby mediates efficient reactivity through delocalized polarity effects. A simplified means of probing these reaction

CHEMISTRY In the search for a practical mode of hydrogen storage for vehicle applications, light metal hydride compounds such as LiBH4 are appealing because of their high weight percentage of hydrogen. However, many such compounds are thermodynamically quite stable, and so require excessive heating

Chemistry What's the strongest acid of them all? In the case of Bronsted acidity, associated with the tendency to lose a proton, it depends on the surrounding medium. The p K a scale was originally grounded in reactivity toward water, but many common solvents are inherently less basic, which allows discrimination of acids that would otherwise all dissociate fully under aqueous conditions. Halogenated carboranes have emerged as the weakest class of conjugate bases in this context; unlike antimony pentafluoride-derived superacid conjugates, they lack a highly Lewis acidic component. Nava et al. report the synthesis and isolation of an acid with the fluorinated carborane CHB11F11 as its conjugate, following up on a report several years ago of its preparation and the vibrational spectrum of an ethyl-substituted analog. Because this compound is sufficiently acidic to cleave alkyl C-H bonds by protonolysis, it was crucial to exclude organic impurities (as well as water) during the steps prior to its ultimate purification by sublimation. Suspension of the solid in hexane was observed to liberate hydrogen at room temperature in 50% yield within 2 hours, a substantial rise in reactivity relative to the previous Bronsted champion, which bore a chlorinated carborane conjugate; butane reacted similarly. Angew. Chem. Int. Ed. 10.1002/anie.201308586 (2013).

Chemistry Widespread use of functional electrocatalytic water-splitting systems in conjunction with solar power infrastructure will probably require a composition of Earth-abundant materials that remain stable under the reaction conditions and operate efficiently at minimal overpotential (a measure of how much of the input energy gets stored in the products' chemical bonds for later use when the Sun has set, rather than lost as heat). Numerous reports highlight favorable features of particular experimental catalysts in this context, but cross-comparisons have proven challenging in the absence of established standards for the testing conditions. McCrory et al. offer a preliminary framework for making apples-to-apples comparisons of catalysts for the oxygen-evolving half of the couple. They report benchmark tests of nine representative non–precious metal catalysts deposited on glassy carbon electrodes, under standardized acidic (1 M sulfuric acid) and basic (1 M sodium hydroxide) conditions. After first assessing the active surface area for normalization, the authors measured the overpotential necessary to attain a current density of 10 mA/cm2 and then tracked performance over 2 hours. None of the tested non–precious metal catalysts were stable in acid, highlighting a priority for further research. In base, the overpotentials across the test set varied relatively little, spanning a range between 350 and 430 mV. J. Am. Chem. Soc. 135 , 16977 (2013).

Chemistry![Figure][1] CREDIT: M. CHRISTINA WHITE The abundance of aliphatic C-H bonds in organic molecules poses an enticing, yet maddening challenge to synthetic chemists. On the one hand, direct oxidation protocols are prospectively applicable to an immense range of substrates; on the other hand, genuinely useful methods must achieve selectivity among numerous sites in a given substrate that differ only subtly. Gormisky and White tackle this challenge through the use of a pair of complementary ligands on an iron catalyst that activates peroxide for C-H oxidation. Elaborating on a previously reported ligand, they introduce bulky bis(trifluoromethyl) phenyl groups that roughly halve the conical angle-bounding substrate approach to the metal center. As a result, this bulkier catalyst favors oxidation at sterically unhindered sites on a substrate, whereas the previous catalyst manifested selectivity governed by the substrate's inherent reactivity. The authors develop a predictive model for the site preferences of each ligand based on steric and electronic factors, and they validate it through site-selective oxidations of such complex molecules as artemisinin (where they can select between two of nine possible sites) and the pentacyclic triacetoxytricalysiolide. J. Am. Chem. Soc. 135 , 10.1021/ja407388y (2013). [1]: pending:yes

Chemistry![Figure][1] CREDIT: MEYER ET AL., NAT. CHEM. BIOL. 9 , 488 (2013) Over 50 years ago, Ronald Breslow suggested that thiamin may adopt a short-lived reactive carbene structure—with a divalent carbon center stabilized by flanking sulfur and nitrogen—during the enzymatic reaction cycle of vitamin B1. In the interim time, chemists have synthesized a huge range of stabilized cyclic carbenes, most of which are derived from imidazole (with two flanking nitrogens) rather than thiazole frameworks. These N-heterocyclic carbenes, or NHCs, are frequently stable and possible to isolate and have been applied as ligands in transition-metal catalysts. They are also versatile catalysts in their own right. Nonetheless, the question of whether thiamin itself reacts as a catalytic carbene in native context has remained open. Meyer et al. present a high-resolution (1.06 A) crystal structure of pyruvate oxidase with bound phosphate (an unreactive analog of the pyruvate substrate), and the associated thiamin molecule shows clear evidence of carbene character. The carbon center in question appears to be deprotonated while lacking the excess electron density indicative of a carbanion. Nat. Chem. Biol. 9 , 488 (2013). [1]: pending:yes

Applied Physics Semiconducting nanocrystals, or quantum dots, are promising materials for use in laser, light-emitting diode (LED), and solar cell applications, partly because of the tunability of their photophysical properties with size. One important size-dependent property in this context is the

Chemistry![Figure][1] CREDIT: NASA Hydration of sulfur trioxide (SO3) to sulfuric acid (H2SO4) in Earth's atmosphere gives rise to acid rain as well as particle formation. A sound mechanistic understanding of this reaction is thus central to the chemistry of climate and pollution mitigation

Chemistry Forty years ago, Polanyi laid out a framework to explain and predict the relative effectiveness of translational and vibrational energy in driving the reactions of atoms with diatomics. The current frontier in state-resolved molecular dynamics research encompasses the reactions of halogen atoms with methane and its various isotopologues—a six-atom system. Over the past several years, successive experimental and theoretical studies have sought to clarify the extent to which the Polanyi Rules apply (or fail to apply) to such systems. For the reaction of chlorine with CHD3, the Rules suggest that the product-like transition state ought to favor the efficacy of vibration over translation, although the results of recent studies have been ambiguous in this regard. Zhang et al. now report quantum dynamics simulations of this reaction and compare their results with those from previous experiments and quasiclassical trajectories. The simulations suggest that C–H stretch excitation does indeed promote the reaction, except at collision energies below 1 kcal/mol. J. Phys. Chem. Lett. 10.1021/jz301649w (2012).

Chemistry In qualitative terms, chemists have become adept at predicting how reaction probabilities depend on the relative dispositions of two colliding reagents. For instance, if a reactive site is bounded by a large fragment and a small one, the collision partner is more likely to make it past the small one. This steric effect is the basis for much of asymmetric catalysis. A more detailed, quantitative picture of what happens at any particular collision angle is rather harder to capture. Wang et al. have now mapped out the precise three-dimensional steric constraints guiding an elementary chemical reaction: chlorine's abstraction of a hydrogen atom in the vibrationally excited C-H bond of CHD3 to produce HCl. By varying the polarization of the vibrational excitation laser, the authors could effectively choose the alignment of the (partially deuterated) methane reagent relative to the incoming Cl atom beam, after which they used ion imaging to map out the product trajectories and their associated quantum states. The results for the production of vibrationally relaxed HCl are largely in keeping with expectations from the longstanding line-of-centers model in collision theory, whereas pathways leading to vibrationally excited HCl manifest more complex dynamics. Nat. Chem. 4 , 636 (2012).

Chemistry Photoinduced electron transfer is the initiating step in photosynthesis, so in a broad sense, it's been key to carbon-carbon bond formation since long before chemists (or humans more generally) arrived on the scene. Present-day synthetic organic chemistry rarely relies on that mechanism, though over the past several years its versatility has been coming into renewed focus. More specifically, catalysts such as tris(bipyridyl)ruthenium can easily lend out an electron after absorbing blue light, and substrates in the electron transfer chain often react differently than they would in a more conventional thermal context. DiRocco and Rovis apply this light-accelerated technique to one half of a dual catalytic cycle, oxidizing an amine to a more reactive iminium ion. A second catalyst—an asymmetrically substituted N-heterocyclic carbene—activates an aldehyde toward nucleophilicity in a dark portion of the cycle, ultimately linking the acyl carbon to the α carbon of the amine. The reaction couples a range of aldehydes to substituted tetrahydroisoquinolines, in many cases with enantiomeric excesses above 90%. J. Am. Chem. Soc. 134 , 10.1021/ja3030164 (2012).